Polarity of Bond

Polarity of Bond

The existence of a hundred percent ionic or covalent bond represents an ideal situation. In reality no bond or a compound is either completely covalent or ionic. Even in case of covalent bond between two hydrogen atoms, there is some ionic character.

When covalent bond is formed between two similar atoms, for example in H₂, O₂, Cl₂ N₂ or F₂, the shared pair of electrons is equally attracted by the two atoms. as a result electron pair is situated exactly between the two identical nuclei. The bond so formed is called nonpolar covalent bond. Contrary to this in case of a heteronuclear molecule like HF, the shared electron pair between the two atoms gets displaced more towards fluorine since the electronegativity of fluorine is far greater than that of hydrogen. The resultant covalent bond is a polar covalent bond.

As a result of polarization, the molecule possesses the dipole moment (depicted below) which can be defined as the product of the magnitude of the charge and the distance between the centres of positive and negative charge. It is usually designated by a Greek latter ‘ ‘. Mathematically, it is expressed as follows:

Dipole Moment (μ) = charge (Q) distance of separation (r)

Dipole moment is usually expressed in Debye units (D).  The conversion factor is

1D = 3.33564   10^-30 Cm,      where C is coulomb and m is meter.

Further dipole moment is a vector quantity and is depicted by a small arrow with tail on the positive centre and head pointing towards the negative centre. For example the dipole moment of HF may be represented as:

The shift in electron density is symbolized by crossed arrow above the Lewis structure to indicate the direction of the shift.

In case of polyatomic molecules the dipole moments not only depend upon the individual dipole moments of bonds known as bond dipoles but also on the spatial arrangement of various bonds in the molecule. In such case, the dipole moment of a molecule is the vector sum of the dipole moments of various bonds. For example in H₂O molecule, which has a bent structure, the two O-H bonds are oriented at an angle of 104.5^o. Net dipole moment of 6.17   10^-30C m (1D=3.33564  10^-30 C m) is the resultant of the dipole moments of two O-H  bonds.

Net Dipole moment,  = 1.85 D  = 1.85   3.33564  10^-30 C m = 6.17 10^-30 C m

The dipole moment in case of BeF₂ is zero. This is because the two equal bond dipoles point in opposite directions and cancel the effect of each other.

In tetra-atomic molecule, for example in BF₃, the dipole moment is zero although the B – F bonds are oriented at an angle of 120^o to one another, the three bond moments give a net sum of zero as the resultant of any two is equal and opposite to the third.  

Let us study an interesting case of NH₃ and NF₃ electrons on nitrogen atom. Although fluorine is more electronegative than nitrogen, the resultant dipole moment of NH₃(4.90   10^-30 C m) is greater than that of NF₃ (0.8  10^-30 C m). This is because, in case of NH₃ the orbital dipole due to lone pair is in the same direction as the resultant dipole moment of the N – H bonds, whereas in NF₃ the orbital dipole is in the direction opposite to the resultant dipole moment of the three N –F bond moments, which results in the low dipole moment of NF₃ as represented below :

Just as all the covalent bonds have some partial ionic character, the ionic bonds also have partial covalent character. The partial covalent character of ionic bonds was discussed by Fajans in terms of the following rules:

The smaller the size of the cation and the larger the size of the anion, the greater the covalent character of an ionic bond.

The greater the charge on the cation, the greater the covalent character of the ionic bond.

For cations of the same size and charge, the one, with electronic configuration          

(n-1)dⁿns^o, typical of transition metals, is more polarising than the one with a noble gas configuration, ns² np⁶,  typical of alkali and alkaline earth metal cations.

The cation polarizes the anion, pulling the electronic charge towards itself and thereby increasing the electronic charge between the two. This is precisely what happens in a covalent bond, i.e., buildup of electron charge density between the nuclei. The polarizing power of the cation, the polarisability of the anion and the extent of distortion (polarization) of anion are the factors, which determine the per cent covalent character of the ionic bond.